Derive Clone for PairSigner (#184)

* Derive `Clone` for `PairSigner`

Signed-off-by: koushiro <koushiro.cqx@gmail.com>

* Specify the date of nightly toolchain to fix CI

Signed-off-by: koushiro <koushiro.cqx@gmail.com>

src/extrinsic/signer.rs

@@ -54,7 +54,7 @@ pub trait Signer<T: Runtime> {
 }
 
 /// Extrinsic signer using a private key.
-#[derive(Debug)]
+#[derive(Clone, Debug)]
 pub struct PairSigner<T: Runtime, P: Pair> {
     account_id: T::AccountId,
     nonce: Option<T::Index>,